Field of the Invention
The invention relates to conveying hollow bodies, in particular preforms made of plastic material (for example, polyethylene terephthalate or PET), in a container manufacturing line.
Description of the Related Art
A container made of plastic material is conventionally formed by blow molding or stretch blow molding starting from a preform that comprises an essentially cylindrical body that is closed by a hemispherical bottom and, at the other end, an open neck, which is separated from the body by a collar and has its final shape.
To allow the forming of the container, the body and the bottom of the preform are subjected to a heating to soften them by bringing them to a temperature (so-called heating temperature) that is higher than the glass transition temperature of the material. In the case of the PET, which has a glass transition temperature of approximately 80° C., the heating temperature is generally approximately 120° C.
The heating of the preform is ordinarily carried out in a furnace that is equipped with infrared radiation sources (generally halogen lamps). The preform is mounted on a revolving support called a “spinner,” provided at one end with a mandrel that is forced into the neck and at another end with a pinion that engages a rack. The spinner moves the preform in front of the lamps, while driving it in rotation to expose the entire body to radiation.
The cutaway view of FIG. 1 illustrates the structure of a known spinner 100, which is installed on production lines during operation.
This spinner 100, mounted (in pairs) on a link 200 of a transport chain, comprises:                A steel shaft 110, mounted in rotation in relation to the link 200 and at an upper end of which a pinion 120 is attached,        An ejector 130 that is stationary in relation to the link 200, and        A mandrel 140 that is integral with the shaft 110.        
The mandrel 140 includes:                A steel crosspiece 150 that is screwed into a lower end of the shaft 110,        An aluminum radiator 160 that is forced onto the crosspiece 150,        An end fitting 170 that is fixed by screwing to the crosspiece 150, which is inserted between the radiator 160 and the end fitting 170,        A radially expandable ring 180, mounted between the end fitting 170 and the crosspiece 150, and        An elastic seal 190 that forces the radial expansion of the ring 180.        
As can be seen in FIG. 1, a first impression 151 is formed in the crosspiece 150 for accommodating a tool such as a screwdriver or an Allen key that makes it possible to ensure the screwing of the crosspiece 150 into a threaded cross-section 152 of the shaft 110. A second impression 171 is, furthermore, formed in the end fitting 170 for accommodating a tool (in this case, an Allen key) that makes it possible to ensure the screwing of the end fitting 170 into a threaded cross-section 172 of the crosspiece 150. As can also be seen in FIG. 1, the two impressions 151, 171 are coaxial, and the second impression 171 is of a size greater than the first impression 151 to make possible the free passage of the tool that corresponds to the former.
A spinner with a similar structure is presented in the U.S. Pat. No. 7,008,215 (Graham Packaging).
The function of the radiator 160, equipped with a series of fins 161, is to limit the heating of the mandrel (and therefore of the neck of the preform that is forced on above) by heat exchange with the ambient air. Without this, the accumulation of calories at the level of the mandrel would have harmful consequences on the production line. First of all, too significant an expansion of the end fitting (under the action of the heat) is able to bring about its wedging in the neck of the preform. Then, taking into account the thermal inertia of the metals, the end fitting, the crosspiece, and the expandable ring, all three in contact with the neck of the preform are able to deform the former by transferring to it a portion of the calories thereof.
To prevent this phenomenon, it was determined that the temperature of the mandrel should not exceed the boundary temperature of 60° C. However, it was noted that, for a heating temperature of 120° C., this boundary temperature is sometimes exceeded, and the above-mentioned consequences occur.